There is a constant concern about the transmission of bacteria and viruses by means of the water in swimming pools, spas, and recirculating systems where water can create an aerosol, such as in fountains and evaporative cooling systems. E. coli bacteria is a major source of contamination in swimming pools, and water used in evaporative coolers is suspected as an agent for the transmission of bacteria that causes Legionnaire's disease (Legionella pneumophila). Typically, water in such systems is processed in a cycle which involves: storage in a reservoir; recirculation into a recirculation unit for filtering, heat exchange, quality monitoring, and chemical addition; and then return to the reservoir. The reservoir typically is open to the environment or at least to chronic sources of contamination, such as human bodies, airborne dust, insects, ornamental plant and tree debris, bird feces, and other sources of biological matter from the environment.
A reservoir of water must periodically be tested and treated with bactericidal chemicals in order to maintain sanitary conditions. This is especially important in swimming pools and spas where bathers have prolonged exposure to water-borne bacteria and viruses. It is well-known that a proper level free of halogens, typically chlorine or bromine, in concentrations of 0.1 parts per million (ppm) to 10 ppm provide strong oxidizing action which is effective in killing bacteria, see White, G. C., Handbook of Chlorination, 2nd Ed., Van Nostrand Reinhold, 1986. Wall, U.S. Pat. No. 4,033,871, teaches the chemistry of chlorine and bromine in swimming pool water.
Hypochlorous acid, HOCl, is recognized as the most effective bactericide of the chlorine residual fractions. In order to provide effective sanitation, the pool and spa industry typically recommends that between 1.0 ppm and 3.0 ppm HOCl be maintained in a swimming pool and between 3.0 to 5.0 ppm HOCl be maintained in a spa. HOCl in water remains in equilibrium with its dissociated form, the hypochlorite ion (OCl.sup.-) and the hydrogen ion (H.sup.+), and their equilibrium balance is strongly effected by the pH of the water. OCl.sup.- is significantly less effective than HOCl in bactericidal action. Therefore, pH must be controlled at proper levels to achieve a useful concentration of HOCl. Low pH (acidic) drives the equilibrium to high concentrations of HOCl, but is an irritant to swimmers as well as accelerating the dissolution of materials in the reservoir walls and the corrosion of metallic components in the water recirculation, heat exchanger, and filtration system, see McMillan, G. K., pH Control, Instrument Society of America, 1984. To avoid irritation, pool and spa industry guidelines suggest keeping the pH of the water between 7.4 and 7.6, preferably at 7.5, although a pH from 7.2 to 7.8 is allowable. At a pH of 7.4, the concentration of HOCl is approximately 60% and the OCl.sup.- concentration is 40%. At a pH of 7.5, the concentrations of HOCl and OCl.sup.- are about equal, and at a pH of 7.6, the HOCl concentration is about 40%. At higher pH, the concentration and hence the effectiveness of HOCl diminishes and the buildup of calcium and other salts can scale pipes, heat exchangers, and sensors.
Therefore, the pH should be maintained in the narrow range of 7.4 to 7.6 and the HOCl residual between 1.0 to 3.0 ppm in typical swimming pool applications. Several investigations have shown that the HOCl residual can be characterized by the oxygen reduction potential (ORP) of the water, but a number of factors, such as the concentration of cyanuric acid, and the type and quantity of total dissolved solids affect the correlation of ORP with ppm of HOCl. Therefore, an ORP set-point for a control system is typically chosen by stabilizing the water at pH 7.5 and a free chlorine residual of 1.0 to 3.0, and measuring the resulting ORP. Thereafter, as the total dissolved solids increase, the control system set-point may become invalid, requiring an adjustment or a water change when the error becomes noticeable to the users.
Both pH and chlorine concentrations present significant nonlinear control problems to an automatic control system when these variables must be maintained within a narrow range for bather comfort, bactericidal effectiveness, and recirculation system reliability. Therefore, there has been a need for a control system that accomplishes consistently precise regulation to save significant expense to the operator in terms of lower maintenance costs and minimal chemical use (by avoiding over- and underdosing), as well as providing a pleasant and comfortable environment for bathers.
A number of automatic chlorine and pH control systems have been developed and marketed which uses sensors to measure the ORP and pH levels, compare the levels to preset reference values, and add an amount of chemical, usually liquid chlorine and muriatic acid. For example: Goudy, U.S. Pat. No. 4,688,699; Newton, U.S. Pat. No. 4,657,670; O'Leary, U.S. Pat. Nos. 4,648,043; 4,550,011; Steininger, U.S. Pat. No. 4,224,154; Severin, U.S. Pat. No. 4,016,079; and Maroney, U.S. Pat. No. 3,554,212 disclose automatic pool chemical control systems. The above systems, and other similar automatic pool chemicals control systems, use analog circuits and timer systems to control chemical dosage of acid and chlorine or bromine. Some include warning lights to indicate an out-of-range condition of pH and ORP.
As is apparent from the above, proper operation of the sensors is critical to proper operation of the systems. However, because the sensors are continually immersed in water, deposits form on them which over time compromise their ability to accurately measure halogen and pH levels.
Specifically, with variations in temperature, alkalinity or increases in dissolved solids like calcium carbonate in pool water, these solutes precipitate and form deposits along immersed surfaces. Although much attention has been directed to improving control systems, little if any has been directed to enhancing the performance of sensors and chemical injectors by avoiding or reversing the build up of deposits on them.
Schleimer, U.S. Pat. No. 3,592,212 addresses the problem of deposit formation in the context of a system for control of scaling (deposit formation) and corrosion in water cooling systems using heat exchange elements. For deposit control, two approaches are suggested to control the saturation point of troublesome dissolved solids. The first is to change the water when the level of dissolved solids gets too high and the other is to add acid to the water when it becomes alkaline so that the solids remain dissolved. The acid addition is done at a location remote from the sensors and although it may retard deposit formation, it may be inadequate to avoid formation and ineffective in removing deposits once formed. Specifically, when acid in added to either the pool or spa or a portion of water removed therefrom, the concentration of acid at the point where it is added increases immediately, but the pH of the remainder of the body of water is affected much less due to the efficacy of dilution.
Discussions with swimming pool installers and maintenance personnel indicate that available control systems for automatic pH and chlorine control are unsatisfactory, generally requiring frequent, on site attention to adjust control set-points and timer duty cycles with changes in bather demand and weather, to discover system operational problems, and to verify reliable sensor operation. The available systems lack the sophistication to address several other important issues. They lack the ability to provide precise control of the chemical dose based on titration curves for each chemical, the ability to automatically adapt to changing conditions affecting chlorine demand, the ability to validate reliable operation of the pH and ORP sensors, the ability to continuously monitor the process and chemical supplies, the ability to set and adjust operating parameters remotely, and the ability to use the injected chemicals to physically maintain the sensors and chlorine injectors of the system in clean working order. The ability to remotely monitor, test, and adjust the operation of an automatic control system using a modem connected to standard telephone lines has been implemented for various types of computer and communications equipment, but such has not been applied to swimming pool and the like water monitoring systems. For example, Attallah, U.S. Pat. No. 5,119,412 discusses a method of remote switching and/or regulating an electrically operated device using DTMF telephone and modem signals and Lamp, U.S. Pat. No. 5,132,904 discloses a remotely accessible microcomputer control system for gas-and oil well heads.